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Old 15th May 2019, 11:59 PM   #1
woody is offline woody
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Default Zen9 question

I have a question about the distortion curve. Although it is unbelievably good for such a simple amp I still have a question. In the graph the modulated cascode curve shows a steady fall in distortion till it gets to about .2 W but then it starts to rise a slight bit why ? I know I am just nit picking and the distortion is so low as to not be important but still wonder why this happens.
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Old 16th May 2019, 02:52 AM   #2
PRR is offline PRR  United States
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This appears to be a THD+N graph: it includes hiss.

It is very typical for a good amp to get down to 0.01%THD near 0.1 Watts, as this does.

What is 0.1 Watts? In 8 ohms it is a little less than 1 Volt.

What is "0.01%"? It is 1/10,000.

So there is "something" at 1/10,000V, or 10 micro-Volts (uV).

If an amplifier has input hiss of 1uV and gain of 10 it has 10uV output hiss.

Why does it rise?

The next line is 0.01 Watts which is 0.08V. Same hiss, lower reference voltage, higher %. 0.03% THD+N at 0.01W actually works out to 24uV, still a *very* reasonable hiss level for almost any gain-amp.

We don't see the left end rise in the not-improved version because the hiss is probably about the same but the THD never comes down below the hiss level, on this chart. We do see it fall-off less. I am sure if the measurements were taken to 0.001W and 0.000,1W we would find a rise.

At the extreme: if we have ZERO signal, thus zero distortion, we still always have hiss. "some hiss" divided by "zero signal" is "infinite THD+N".
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Old 16th May 2019, 04:11 AM   #3
rothko is offline rothko
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What kind of equipments are used to get that curve? Is it measured using real speaker as a load? Or any other load are used instead og real speakers?
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Old 16th May 2019, 12:50 PM   #4
woody is offline woody
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PRR I think you are right it must just be hiss. What threw me is that it's probably only because the distortion is so low that the effect of hiss shows up at very low output.
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Old 16th May 2019, 04:10 PM   #5
Nelson Pass is offline Nelson Pass  United States
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Zen9 question
More hum than hiss.
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Old 16th May 2019, 04:11 PM   #6
Nelson Pass is offline Nelson Pass  United States
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Zen9 question
Quote:
Originally Posted by rothko View Post
What kind of equipments are used to get that curve? Is it measured using real speaker as a load? Or any other load are used instead og real speakers?
8 ohm resistor for that curve.
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Old 16th May 2019, 11:13 PM   #7
Loudthud is offline Loudthud  United States
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Quote:
Originally Posted by rothko View Post
What kind of equipments are used to get that curve? Is it measured using real speaker as a load? Or any other load are used instead og real speakers?
I think Nelson is using an Audio Precision System 1.

Link: How Distortion Free are the Distortion Measurers?

In reply 30 of the thread linked above I posted a graph of what you get if you connect the generator of a System 1 to it's analyzer.
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